1. Field of the Invention
The present invention relates to a bath for immersion tin-lead plating on copper or copper alloys used for electrodes of various electronic circuit elements.
2. Prior Art
Copper and copper alloys are widely used for various electrodes or lines of printed circuit boards packaging electronic circuit elements. The electrodes or the circuit lines may be plated with tin. In the course of tin-plating, however, whiskers are frequently observed as shown in Reference 1 and FIG. 7, which may cause a short-circuit.
In tin-lead plating, on the other hand, whiskers are observed less frequently and to a less extent; especially when the content of lead exceeds 5%, whiskers are substantially prevented. Tin-lead plating attains high corrosion resistance and excellent solderability, thus being applied to variety of electrodes or circuit lines so as to improve their quality and to simplify the manufacturing process.
Electroplating is a general practice for plating a substance with a tin-lead alloy. Both the anode and the cathode, the substance, are dipped in an electrolytic solution including stannic or stannous ions and lead ions; electric current is then supplied to the solution to plate the substance with a tin-lead alloy.
In electroplating, however, current density varies depending on the region of the substance; the thickness of the plating layer is thus not uniform. Furthermore, electrically insulated substances cannot be electroplated.
The advanced electronics technology attains compact designs and multi-functions and high performance of electrical appliances. A highly efficient plating method is thus highly required for plating circuit boards, semiconductors, etc.
Immersion plating method is adopted for the above purpose. Immersion plating does not require electricity for plating, and thereby any substance can be plated irrespective of the shape and electric conductivity.
For immersion plating, acidic immersion-plating bathes are prepared by mixing thiourea, fluoroboric acid, a chelating agent and hydrochloric acid with tin chloride, lead acetate, tin fluoroborate and lead fluoroborate. As shown in Reference 2, when the bath contains a relatively high concentration of chloride ions, lead chloride-thiourea complexes precipitate, which are sparingly soluble at a room temperature. Since the precipitate prevents plating, the bath should be maintained at a higher temperature. Otherwise, the complexes are suspended in the bath and easily deteriorate the bath.
When the substance is wet, water drops attached to the substance partially lower the temperature of the plating bath; as a result, the above complexes are precipitated to prevent the substance from being plated uniformly. Lead deposits more easily than tin, and thus a desirable composition of tin-lead plating is difficult to be obtained.